Oxygen-17 Nmr Studies of Uranium (VI) Hydrolysis and Gelation
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OXYGEN-17 GELATION
NMR
STUDIES
OF URANIUM
(VI)
HYDROLYSIS
AND
R. BRUCE KING*, CHARLES M. KING**, AND A. RONALD GARBER***
*Department of Chemistry, University of Georgia, Athens, Georgia 30602 "**Westinghouse Savannah River Company, Savannah River Site, Aiken, South Carolina, 29802 ***Department of Chemistry, University of South Carolina, Columbia, South Carolina 29201 ABSTRACT Hydrolysis and gelation processes in uranyl solutions are observed using the strong sharp uranyl oxygen-17 resonance. INTRODUCTION The hydrolysis and subsequent gelation of uranyl ion is of both fundamental and practical interest. From a fundamental point of view the uranyl and other actinyl ions are unusual in having inert strong multiple bonds to the axial oxygen atoms and labile weak bonds to the equatorial ligands. From a practical point of view the hydrolysis and subsequent gelation of uranyl ion is important in connection with the manufacture of U02 microspheres for nuclear fuel applications [1]. This paper describes our efforts to follow the hydrolysis and gelation of uranyl derivatives by using oxygen-17 NMR spectroscopy of the axial oxygens as a probe. The oxygen-17 resonance of the axial uranyl oxygens, which was first observed by Rabideau [2], is very narrow and thus very easy to observe relative to other types of resonances of the quadrupolar oxygen-17 nucleus. In addition the position of the oxygen resonance in uranyl derivatives is very sensitive to the equatorial ligands [31 and even to isotopic substitution of the opposite uranyl oxygen atom [4]. Furthermore the kinetic inertness of the axial uranyl oxygen to oxygen exchange [5] allows distinct resonances for different types of uranyl groups to be observed in mixtures of uranyl 'species rather than a single uranyl resonance at an average chemical shift for the components of the mixture. EXPERIMENTAL Oxygen-17 NMR spectra were obtained at 11.7T (67.8 MHz) on a Bruker AM-500 at the Chemistry Department of the University of South Carolina. Data were acquired at a sweep width of 125 kHz following excitation by a 6.2 msec pulse (-66°). Each scan consisted of 65,536 (64K) data points. Minimum relaxation delays were used giving a 0.262 sec recycle time. The resulting FID' s were zero filled to 128K before transformation yielding a digital resolution of 1.9 Hz/point. The spectra in the Figures are the sum of 4096 individual scans. Chemical shifts are reported relative to H20 and were referenced to an external D20/H20 standard. All samples were run in 5 mm NMR tubes and spectra were acquired at ambient temperature. Concentrated (1.8 to 3.9 M) aqueous stock solutions of uranyl chloride and uranyl nitrate were enriched photolytically [5] to 5 to 10% oxygen-17 using commercial 28% enriched oxygen- 17 water purchased from Icon Services, Inc. (Summit, New Jersey). RESULTS Figure 1 shows the oxygen-17 NMR spectra in the uranyl region obtained by the 2 hydrolysis of uranyl chloride by the addition of 10% aqueous Me4NOH in 0.25 OH-/UO2 ÷ 2 increments. The final spectra clearly show the
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